Electronically controllable switches, or briefly “switches”, such as transistors, variants thereof and circuits including one or more of these devices are widely used in many different applications. Important examples are switched power supplies, where often MOSFETs (metal-oxide-semiconductor field-effect transistor) are used as the power switches. Two main types of MOSFETs exist: P-Channel MOSFETs and N-channel MOSFETS. Both types need different voltage control signals for proper function.
The signal for controlling the switching of such a MOSFET usually is provided by a controller and a driver circuit (also designated briefly as “driver” in the following) is used to translate the control signal received from the controller into a drive signal for properly driving the MOSFET. This is necessary because the characteristics of a control signal provided by a controller, in particular the provided voltage levels, usually do not match the requirements of the particular type of MOSFET used.
In the case of a P-Channel MOSFET, the driver has to apply a negative voltage to the gate-source junction of the MOSFET to switch the MOSFET ON, where the negative voltage has to be below the threshold voltage of the particular MOSFET. In order to turn the MOSFET OFF, a voltage above the threshold voltage has to be applied to the gate-source junction of the MOSFET. If the threshold voltage of a P-Channel MOSFET is for example −3 Volts, applying a voltage of −10 Volts to the gate-source junction of that MOSFET turns it ON. Applying however a voltage of for example −1 Volts to the gate-source junction of that MO SFET turns it OFF.
Two well known and widely used P-Channel MOSFET drivers are the two-stage gate drivers and the AC-coupled gate drivers. As the name already implies, the two-stage gate drivers include two different stages to provide the drive signal wherefore they have a high degree of complexity. Depending on the particular application, they also have other drawbacks such as for example relatively high power losses and/or switching frequency limitations. The AC-coupled gate drivers have been proposed to reduce the complexity of the driver. The supply voltage is fed through an inverting capacitor to the gate during the turn-on and -off and through a resistor during the on-state. The limitations include for example frequency and duty cycle limitations as the coupling capacitor needs to discharge during turn-off via a high impedance path, a high power loss in the DC current resistor and the need for an external gate-source capacitor. Such a driver circuit is shown in the drawings and described further below. Such AC-coupled driver circuits are easy to implement and cost effective.
However, they are not immune against voltage build-up directly between the gate and the source of the P-Channel MOSFET due to charge displacement or capacitive coupling during the turn-off period. The main problem comes from the fact that the inverting capacitor constitutes a high DC impedance between the driver and the gate of the P-Channel MOSFET. This high DC impedance is what enables a DC voltage to build-up in the gate of the P-Channel MOSFET. Such a voltage build-up can lead to undesired turn-on of the MOSFET which in turn may lead to a malfunction or even damage of the higher level circuit that includes the MOSFET.